Selective placement of carbon nanotubes via coulombic attraction of oppositely charged carbon nanotubes and self-assembled monolayers

ABSTRACT

A method of forming a structure having selectively placed carbon nanotubes, a method of making charged carbon nanotubes, a bi-functional precursor, and a structure having a high density carbon nanotube layer with minimal bundling. Carbon nanotubes are selectively placed on a substrate having two regions. The first region has an isoelectric point exceeding the second region&#39;s isoelectric point. The substrate is immersed in a solution of a bi-functional precursor having anchoring and charged ends. The anchoring end bonds to the first region to form a self-assembled monolayer having a charged end. The substrate with charged monolayer is immersed in a solution of carbon nanotubes having an opposite charge to form a carbon nanotube layer on the self-assembled monolayer. The charged carbon nanotubes are made by functionalization or coating with an ionic surfactant.

PRIORITY

This application is a continuation of and claims priority from U.S.patent application Ser. No. 13/248,176, filed on Sep. 29, 2011, entitled“SELECTION PLACEMENT OF CARBON NANOTUBES VIA COULOMBIC ATTRACTION OFOPPOSITELY CHARGED CARBON ANNAOTUBES AND SELF-ASSEMBLED MONOLAYERS”, theentire contents of which are incorporated herein by reference.

BACKGROUND

The present invention generally relates to a method of placing carbonnanotubes (herein after “CNTs”) on a substrate. In particular, theinvention relates to selective placement of charged CNTs on apre-patterned surface having an oppositely charged, self-assembledmonolayer.

CNTs can be semiconducting and therefore are of interest as channelmaterials for Field Effect Transistors (herein after “FET”).Accordingly, methods of placing CNTs on a substrate for use in FETs arebeing explored.

One approach to placing CNTs on a substrate involves directed assemblyof CNTs from a suspension. In this approach, a substrate is patterned todefine areas to which the CNT will have an affinity. The affinity is dueto functionalization of either the substrate or the CNT to promotebonding between the substrate and the CNT.

In one instance, to place CNTs on a substrate, the prior art stamps asubstrate with an organic compound to create a substrate havinghydrophilic and hydrophobic regions. The hydrophilic region is theoriginal substrate surface and the hydrophobic region is the areastamped with the organic compound. The substrate is immersed in asolution of CNTs and dried to leave CNTs on the hydrophilic regions.However, the CNTs on the surface of the substrate are bundled (i.e. agroup of CNTs twisted together in a rope-like fashion) and/ormultilayered. Bundled or multilayered CNTs are undesirable because atransistor made from them requires higher voltage to turn on an off. Thedescribed method has another drawback in that a solution of CNTs is notable to reach recessed hydrophilic areas having small widths (around orless than 200 nm). As a result, CNTs will be placed in large hydrophilicareas while small hydrophilic features remain uncovered. Accordingly, aCNT placement method based upon hydrogen bonding (a type of dipolebonding) can result in poor selectivity.

In other methods, the prior art places CNTs on a substrate by firstfunctionalizing the CNT and then placing the CNT directly on thesubstrate. However, such methods result in a low density of CNTs on thesurface.

Therefore, a need exists for a method of selectively placing a monolayerof high density CNTs on a substrate with minimal bundling.

SUMMARY

An object of the invention is to provide a method of forming a structurehaving selectively placed carbon nanotubes (“CNTs”). The method includesproviding a substrate having a surface and contacting the surface of thesubstrate and a solution of a precursor molecule to form aself-assembled monolayer having a first ionic charge moiety on thesurface. Thereafter, the self-assembled monolayer and a dispersion of aplurality of CNTs having a second ionic charge moiety are contacted.

According to another aspect of the invention a structure having a CNTlayer includes a substrate having a first region and a second region, aself-assembled monolayer on the first region, and a CNT layer on theself-assembled monolayer. The CNT layer has a density exceeding 1 CNTper square micron.

According to a further aspect of the invention, a bi-functionalprecursor molecule for making self-assembled monolayers is disclosed. Abi-functional precursor molecule includes a first functional group toanchor the monolayer to a substrate and a second functional group havinga first ionic charge moiety. The first functional group is selected fromthe following group: a thiol, an isontrile, a phosphonic acid and ahydroxamic acid. The first ionic charge moiety can be an onium saltincluding an ammonium salt, a sulfonium salt, and phosphonium salt.

Advantages of the present invention include increased density ofnanotubes, and reduced formation of multilayer CNTs or bundled CNTs.

Another advantage is better electrical performance of a CNTFET.

Other characteristics and advantages of the invention will becomeobvious in combination with the description of accompanying drawings,wherein the same number represents the same or similar parts in allfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a method of placing CNTs above a substrateaccording to an embodiment of the invention;

FIG. 2a illustrates a cross section of patterned substrate according toan embodiment of the invention;

FIG. 2b illustrates a top down view of the patterned substrate of FIG.2a according to an embodiment of the invention;

FIG. 3 illustrates bi-functional precursor material forming aself-assembled monolayer on regions of the substrate having a firstisoelectric point according to an embodiment of the invention;

FIG. 4 illustrates a self-assembled monolayer with a first ionic chargemoiety on the first region of the patterned substrate contacting asolution of CNTs having a second ionic charge moiety according to anembodiment of the invention;

FIG. 5 illustrates a CNT layer on a self-assembled monolayer formed by amethod according to an embodiment of the invention;

FIG. 6 is a scanning electron microscope image of substrate with a layerof selectively placed CNTs according to an embodiment of the invention;

FIG. 7a illustrate a bi-functional precursor group having a firstfunctional group and a second functional group which is an ammonium salthaving a first ionic charge moiety according to an embodiment of theinvention wherein the first ionic charge moiety is positively charged;

FIG. 7b illustrate a bi-functional precursor group having a firstfunctional group and a second functional group which is an ammonium salthaving a first ionic charge moiety according to an embodiment of theinvention wherein the first ionic charge moiety is positively charged;

FIG. 7c illustrate a bi-functional precursor group having a firstfunctional group and a second functional group which is a pyridiniumsalt having a first ionic charge moiety according to an embodiment ofthe invention wherein the first ionic charge moiety is positivelycharged;

FIG. 7d illustrate a bi-functional precursor group having a firstfunctional group and a second functional group which is a sulfonium salthaving a first ionic charge moiety according to an embodiment of theinvention wherein the first ionic charge moiety is positively charged;

FIG. 7e illustrate a bi-functional precursor group having a firstfunctional group and a second functional group which is a phosphoniumsalt having a first ionic charge moiety according to an embodiment ofthe invention wherein the first ionic charge moiety is positivelycharged;

FIG. 8a illustrates a first step in the method of forming a CNT having asecond ionic charge moiety by functionalizing a CNT according to anembodiment of the invention, in the first step an organic salt is adiazonium salt (R—N₂ ⁺X⁻) is combined with a CNT dispersion to form afunctionalized CNT shown in FIG. 8 b;

FIG. 8b illustrates a second step in a method of forming a CNT having asecond ionic charge moiety by functionalizing a CNT according to anembodiment of the invention, in the second step KOH and water are mixedwith the functionalized CNT to form the final product shown in FIG. 8 c.

FIG. 8c illustrates the final product, the CNT having a second chargemoiety, in a method of forming a CNT having a second ionic charge moietyby functionalizing a CNT according to an embodiment of the invention;and

FIG. 9 illustrates a method of forming a CNT having a second ioniccharge moiety by coating a CNT with an ionic surfactant according to anembodiment of the invention;

DETAILED DESCRIPTION

The basic principle of the invention is a method for forming a highdensity monolayer of CNTs on a substrate with little or no bundling. Themethod, as well as the resulting structure, will be described inconjunction with FIGS. 1-6. The invention also includes a bi-functionalprecursor molecule for self-assembled monolayers used in the method ofplacing the CNTs on a substrate. An embodiment of the bi-functionalprecursor is described in conjunction with FIG. 7. The invention furtherincludes methods for creating CNTs having a second ionic charge moietyembodiments of which are described in conjunction with FIGS. 8-9. Adetailed description of the invention is made in combination with thefollowing embodiments. Please note that reference numbers are merelyreference numbers and, thus, do not necessarily restrict the method tothe numerical order of the reference numbers.

Referring to FIG. 1, a flow chart of a method of placing CNTs above asubstrate according to an embodiment of this invention is given. Atreference numeral 10, a patterned substrate is provided. Patterning thesubstrate creates a first region and a second region on the substrate.At reference numeral 20, the patterned substrate is put in contact witha solution containing a precursor. The precursor is bi-functional,meaning it has two functional groups which serve two different purposes.The first functional group serves to anchor the precursor to thesubstrate and the second functional serves as (first) ionic chargemoiety. By contacting the substrate to the precursor solution, the firstfunctional group (the anchoring functional group) forms a bond withfirst region of the substrate thereby forming a self-assembled monolayerat that region; the second functional group remains in place, thus themonolayer has a first ionic charge moiety. At reference numeral 30, thesubstrate with self-assembled monolayer, is put in contact with adispersion containing CNTs having a second ionic charge moiety. Thefirst and second charge moieties are opposites, thus the CNTs areelectrostatically attracted to the self-assembled monolayer having thefirst ionic charge moiety which results in a layer of CNTs on theself-assembled monolayer. At reference numeral 50, the substrate isrinsed to leave a CNT layer above the self-assembled monolayer, which inturn is above the first region of the substrate.

Referring to FIG. 2a , a patterned substrate 200 is shown incross-section. The substrate has a base 210, a first region 220 and asecond region 230. Here, the second region 230 is shown as being higherthan the first region 220, however, the regions could be co-planar orthe first region 220 could be higher than the second 230. Here, thesecond region 230 is shown to be on top of a portion of the first region220. However, the opposite formation could also occur, or the regionscould abut each other. Thus, the exact cross-sectional configuration ofthe regions relative to each other and the base may have many varieties(even beyond those varieties discussed above), of which FIG. 2a is butone embodiment.

Referring to FIG. 2b , the same patterned substrate 200 is shown from atop-down perspective. In FIG. 2b the first region 220 is a simple stripthat is flanked by the second regions 230. However, the first and secondregions may take on a variety of shapes or configurations.

Next, isoelectric properties of the first and second regions arediscussed. An isoelectric point is the pH at which a surface carries nonet electric charge. In this invention, the first region 220 has a firstisoelectric point and the second region 230 has a second isoelectricpoint. The first and second isoelectric points are different from eachother, with the first isoelectric point (i.e. isoelectric point of thefirst region 220) being greater than the second isoelectric point (i.e.isoelectric point of the second region 230). Preferably, the differenceof the isoelectric point of the first region and that of the secondregion should be about four or greater. For example, a first region 220of hafnium oxide having an isoelectric point around 7 and a secondregion 230 of silicon dioxide having an isoelectric point around 2results in a difference in the isoelectric points of about 5.

Next, materials suitable as first and second regions of the patternedsubstrate are discussed. In a first embodiment, the first region 220 isa metal oxide and the second region 230 is a non-metal oxide such as,but not limited to, a silicon oxide (Si_(x)O_(z)H_(z)). The metal oxideincludes at least one metal from group IVB, VB, VIB, VIIB, VIII or IIA(CAS version) of the Periodic Table of the Elements. Illustratively, themetal oxide first region 220 can be an aluminum oxide (Al₂O₃), a hafniumoxide (HfO₂), a titanium oxide (TiO_(x)), or a zinc oxide (ZnO). In asecond embodiment, the first region 220 can be any oxide, includingnon-metal oxides and metal oxides. In the second embodiment, the secondregion 230 is a metal. Examples of metals for use in the second regioninclude gold, palladium, copper, platinum, etc.

Referring to FIG. 3, the patterned substrate 200 comes in contact with asolution 240 containing a bi-functional precursor 250 to form aself-assembled monolayer (herein after “SAM”) 265 having a first ioniccharge moiety 264. The precursor 250 is bi-functional, meaning it hastwo functional groups (252, 254) which, in turn, serve two differentpurposes.

The first functional group 252 serves to anchor the precursor 250 to thefirst region 220 of the patterned substrate 200. The second functionalgroup 254 has a first ionic charge moiety 264 which serves to form abond with a second ionic charge moiety of the CNT later in the process.By contacting the patterned substrate 200 to precursor solution 240, thefirst functional group 252 (the anchoring functional group) forms a bondwith first region 220 of the substrate thereby forming a self-assembledmonolayer 265 having a first ionic charge moiety 264 at the first region220. (Specific examples of bi-functional precursors and the first andsecond functional groups are discussed later).

The first ionic charge moiety 264 can be either positively or negativelycharged. In a preferred embodiment, the patterned substrate 200 has ahafnium oxide first region 220 and a silicon oxide (SiO_(x)) secondregion 230. In a preferred embodiment, the SAM 265 formed on the firstregion 220 (hafnium oxide) has a positive first ionic charge moiety 264.As a result, at that point in the particular preferred embodiment, theentire surface of the substrate is now hydrophilic prior CNT layerformation. This is in contrast to prior art methods relying onhydrophilicity and hydrophobicity differences on a substrate surface todetermine selectivity of CNT placement. Thus, in the present invention,selectivity is, in part, determined by the isoelectric point differencein the first and second regions of the substrate.

Referring to FIG. 4, a SAM 265 with a first ionic charge moiety 264 onthe first region 220 of the patterned substrate 200, contacts a solution270 of CNTs 271 having a second ionic charge moiety 274. Ways offormation of the CNT having a second ionic charge moiety are discussedlater. Coulombic attraction between the oppositely charged first ionicmoiety 264 and second ionic moiety 274 bonds 284 the CNT 271 to the SAM265 in the first region 220 of the substrate (See FIG. 5). The substrateis rinsed in water to form leave a CNT layer 290 selectively formedabove the first region 220 of the patterned substrate 200. The rinsingstep removes any excess CNTs to preferably form a monolayer of CNT. TheCNTs of the layer 209 may be single walled or multi-walled CNTs. Arinsing step is possible in the present invention method because thebond 284 created between the oppositely charged ions of the CNTs and SAMis stronger than a mere hydrogen bond found in prior are methods. Thus,the bond 284 will not dissociate in water like a hydrogen bond can.

An advantage to the present invention that the charge moiety on the CNTis from a charged functionality or charged surfactant around the CNT, asopposed to an induced charge of the nanotube itself. By using a chargedfunctionality or surfactant attached to the CNT, interaction between theCNT and the SAM covered substrate is increased which results inincreased CNT density on the desired region of the substrate 200.

Still referring to FIG. 5, the CNT layer 290 exhibits a density of CNTsfrom about 10 CNT/μm² to about 100 CNT/μm² and ranges therebetween. Theresulting CNT layer 290 exhibits a reduced bundle density of from about0.1 bundles/μm² to about 1 bundles/μm² and ranges therebetween. FIG. 6is a scanning electron microscope image of substrate with selectivelyplaced CNTs according to an embodiment of the invention.

Next, suitable bi-functional precursors 250 will be discussed. As statedearlier, the bi-functional precursor has a first functional group 252for anchoring and a second functional group 254 for forming a firstionic charge moiety 264. The identity of the first functional group (theanchoring group) 252 depends upon the material of the first region 220.When the first region 220 material is a metal, the first functionalgroup 252 is a thiol (—SH) or an isontrile (—NC). When the first region220 material is a metal oxide, the first functional group 252 is aphosphonic acid (—PO₃H₂) or a hydroxamic acid (—CONHOH).

Next, the second functional group 254 of the bi-functional precursor 250will be discussed. The second functional group 254 can be converted tothe ionic charge moiety 264 (also referred to as charged ionic moiety inthis application). The second functional group 254 can be converted toionic charge moiety 264 either (1) before the precursor 250 anchorsitself to the substrate to form a self-assembled monolayer, or (2) afterthe precursor 250 anchors itself to the substrate to form aself-assembled monolayer. In the first case, the self-assembledmonolayer, as formed, has a first charge moiety 264. In the second case,initially the self-assembled monolayer is uncharged, and must beconverted to a SAM having a first ionic charge.

Referring to FIGS. 7a-e , examples of bi-functional precursors 250 inwhich the second functional group 254 has been converted to a firstionic charge moiety 264 having a positive charged. All the bi-functionalprecursors 250 have an “R” group on the bottom which represents thefirst functional group 252 (i.e. the anchoring group) previouslydescribed. In FIGS. 7a-e , the first ionic charge moieties 264 are allpositively charged onium salts. Specifically, FIGS. 7a-b illustrateammonium salts, FIG. 7c is an example of pyridinium salt, FIG. 7d is asulfonium salt, and FIG. 7e is a phosphonium salt. Continuing, in FIGS.7a and 7b , “n” represents an integer from 2 to 16; in FIG. 7b “Z”represents a single bond, oxygen, NH or sulfur; in FIGS. 7a, 7b, 7c and7d , “R₁”, R₂″, and “R₃” can independently be hydrogen, or an alklygroup of one to ten carbons; in FIG. 7e “Ar₁”, “Ar₂”, and “Ar₃” canindependently be phenyl or substituted phenyl rings; and in all FIG. 7,“X” is a halogen. The ammonium salts pictured in FIGS. 7a-b are superiorto a diazonium salt (—N₂ ⁺) because a diazonium salt will form acovalent bond with a CNT (whether the CNT has a partial charge or fullionic charge). Covalent bonds degrade the electrical performance of aCNT in transistor and other electronic applications.

Second functional groups 254 that can be converted to positively chargedmoieties of FIG. 7 are as follows: ammoniums salts (FIGS. 7a and b ) canbe made by reacting an amine with acids; pyridinium salt (FIG. 7c ) canbe made by reacting pyridine with alkyl halides; sulfonium salt (FIG. 7d) can be made by reacting sulfides with alkyl halides; and phosphoniumsalt (FIG. 7e can be made by reacting triarylphosphines with alkylhalides.

While FIGS. 7a-e illustrate examples of a first ionic charge moiety 264having a positive charge, a bi-functional precursor 250 having anegatively charged first ionic charge moiety 264 is also within thescope of the present invention. In the case of a negatively chargedfirst ionic moiety, the bi-functional molecules are the same as FIGS.7a-e with the exception that the positive charge moieties 264 aresubstituted with negative charged moieties such as —COO⁻ or Ar—O⁻.Examples of second functional groups 254 that can be converted tonegatively charged moieties 264 are carboxylic esters such as —COOCH₃,and phenols. These groups are converted to the negatively chargedmoieties 264 by reacting with a strong base such as KOH to yield —COO⁻or Ar—O⁻, respectively.

Next, the term “ionic charge moiety” will be discussed. A moiety is apart of a molecule which has a charge. In the present invention, acharge is formed because one molecule takes an electron, or pair ofelectrons, from another; meaning the charge moiety is ionic. Theionically charged molecule of the present invention should be contrastedto a polar molecule (such as those found in water, —NH₂, —NHNH₂, —ONH₂,—ONHOH, and —CONHO—). Polar molecules are molecules which have unevenelectron distribution in a molecule. Because polar molecules have anuneven electron distribution, an atom in the molecule is sometimesreferred to as partially charged or is said to have a dipole moment.However, the weak bonds which result from oppositely (partially) chargeddipole-dipole attractions, should not be confused with the strongerbonds resulting from the attraction of oppositely charged ions of thepresent invention.

The discussion returns to the two ways of creating the second ioniccharge moiety 274 on the CNTs 271. Functionalization, the first way ofcreating the second ionic charge moiety 274 on the CNT can beaccomplished by mixing an aryl diazonium salt with a CNT dispersion toform a CNT covalently bonded to an organic compound having a functionalgroup. The CNT with functional group is then placed in an aqueous strongbase solution to convert the functional group to the second ionic chargemoiety 274. Examples of strong bases include, but are not limited to,LiOH, NaOH, RbOH, CsOH, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂, and KOH, which ispreferred.

Referring to FIGS. 8a-c , a specific example of functionalizing a CNT toform a negatively charged ionic moiety 274 is shown. In FIG. 8a , theorganic salt is a diazonium salt (R—N₂ ⁺X⁻) which is combined with a CNT271 dispersion. Here, “R” of the diazonium salt is methyl benzoate. Thesalt bonds covalently with the CNT to form a CNT 271 having a functionalgroup (i.e. a “functionalized CNT”) in FIG. 8b . Continuing with FIG. 8b, KOH and water are mixed with the functionalized CNT to convert aportion of the functional group (here, the carboxylic ester —COOCH₃) tothe second ionic charge moiety 274 (here, —COO⁻).

A specific example of functionalizing a CNT to form a positively chargedionic moiety 274 is not shown in FIGS. 8a-c , however, a pyridinediazonium salt can be mixed with a CNT dispersion to form afunctionalized CNT. Then the functionalized CNT is treated either withan acid to form pyridinium salt or with an alkyl halide (for example,but not limited to methyl iodide) to form N-alkylpyridinium halide. Thetreatments convert a portion of the functional group (here, pyridine) tothe second ionic charge moiety 274 (here, CH₃ ⁻X⁺).

Referring to FIG. 9, coating, the second way of creating the secondionic charge moiety 274 is shown. Coating on the CNT can be accomplishedby mixing CNTs 271 with a solution of an ionic surfactant 276 having thesecond charged ionic moiety 274. In FIG. 9, a water dispersioncontaining CNTs 271 is mixed with ionic surfactants 276. Some of theionic surfactant 276 material forms a monolayer around the CNTs as shownin FIG. 9 as CNTs 271 with an overlapping ionic surfactant 276. Thesolution containing the CNT dispersion and ionic surfactant under goesdialysis in pure water to remove any excess ionic surfactant. Byremoving the excess ionic surfactant 276, the placement yield of theCNTs having a second ionic charge moiety on the SAM having a first ioniccharge moiety increases because there is less likelihood that a SAM sitewill be occupied by an ionic surfactant 276 which is not associated witha CNT 271.

The second ionic charge moiety 274 will be oppositely charged from thefirst charged ionic moiety 264. Therefore, if the first ionic chargemoiety 264 is positive, the ionic surfactant 276 will be anionic; if thefirst ionic charge moiety 264 is negative, the ionic surfactant 276 willbe cationic. Examples of anionic surfactants include, but are notlimited to DNA, sodium dodecyl sulfate and sodium cholate, in additionsome lipids and phospholipids are useful as anionic surfactants.Examples of cationic surfactants include, but are not limited to,cetytlpyridinium chloride, diemthyldioctadecylammonium chloride.

The surfactants may be made ionic prior to coating the CNTs 271 or aftercoating the CNT 271. For example, the anionic surfactants can beprepared by treating their acid form with base and the cationicsurfactants can be prepared by treating tertiary amines with alkylhalides.

An advantage of the present invention is that a bond 284 between a twocharged molecules is stronger than a charge between a polar molecule anda CNT. Furthermore, the first ionic charge moieties 264 disclosed aresuperior to alternative moieties (for example, Ar—N₂ ⁺) because thecurrent first ionic charge moieties do not from covalent bonds with theCNT like Ar—N₂ ⁺ does. In some cases, covalent bonds to a CNT mayinterfere with the electronic properties of the CNT. One electricalproperty of a CNT field effect transistor is its transfer characteristic(Id(A) vs. Vg(V)). A low ON current equates to poor performance and isan indicator of a covalent bond to the CNT. A higher ON current (10⁻⁷ Aor more) indicates return of the lattice of the CNT and thus, no tominimal covalent bonding.

Next, example embodiments of the present invention are given.

Example I

Positively charged bi-functional precursor molecules for self-assembly.Potassium cyanide (50 mg) was added to a solution of methylisonicotinate (1.17 g, 0.01 mole) and 50% hydroxylamine in water (1.3 g,0.02 mole) in 10 mL tetrahydrofuran and 5 mL methanol. After stirringthe mixture at room temperature for 18 hours, the precipitate wasfiltered and washed with diethyl ether and dried to give analyticallypure N-hydroxy isonicotinamide. The latter was added to 5% methyl iodidein methanol and stirred at room temperature for two days. Methanol wasevaporated under reduced pressure and the solid residue was crystallizedfrom ethanol resulting in pure 4-hydroxamido-N-methylpyridinium iodide.

Example II

Preparation of negatively-charged CNTs by functionalization Nitrosoniumtetrafluoroborate (12 mg, 1 mmole) was added to a suspension of methyl4-aminobenzoate (15 mg, 1 mmole) in 5 mL of acetonitrile. The resultingsolution was added drop-wise to an aqueous suspension of single-walledcarbon nanotubes (1 mg) in water containing 1% sodium dodecylsulfate.After standing for 18 hours, the solution was centrifuged and thesediments were added to 10 mL of 10% methanolic potassium hydroxidesolution. After stirring for 4 hours, 20 mL of acetone was added and themixture centrifuged. The supernatant liquid was discarded and thesediment was dissolved in de-ionized water resulting in an aqueoussolution of negatively charged carbon nanotubes.

Example III

Preparation of an aqueous dispersion of CNTs coating in with a monolayerof anionic surfactant. Dispersion of carbon nanotubes in 1% sodiumdodecylsulfate was dialyzed with pure water for several days, duringwhich fresh water was used after 24 hours. After several times dialyzingwith fresh water, the solution inside the filter contains no freesurfactant and all surfactants are attached to carbon nanotubes.

Example IV

Selective placement of carbon nanotubes. A silicon substrate patternedwith silicon oxide regions and hafnium oxide regions was immersed in a 2mM solution of 4-hydroxamido-N-methylpyridinium iodide in ethanol. Afterone hour, the substrate was removed from the solution and rinsed withcopious amount of ethanol and dried under stream of nitrogen. Thesubstrate, now coated with a positively charged ionic monolayer on thehafnium oxide, was then immersed in a solution of negatively charged(ionic, not dipole) functionalized nanotubes of example II. After onehour, the substrate was removed and washed thoroughly with de-ionizedwater and dried under stream of nitrogen. Scanning electron microscopy(FIG. 9) of the substrate showed selectively placed high density ofcarbon nanotubes on regions of hafnium oxide.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadcast interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. A structure having a carbon nanotube (CNT) layer,the structure comprising: a substrate having a first region and a secondregion; a self-assembled monolayer on the first region; and a CNT layeron the self-assembled monolayer; wherein the CNT layer has a densityexceeding 1 CNT per square micron.
 2. The structure of claim 1, whereinthe CNT layer has a density of bundled CNTs of less than 1 CNT bundleper square micron.
 3. The structure of claim 1, wherein theself-assembled monolayer comprises a precursor molecule having a firstfunctional group to anchor the monolayer to the first region of thesubstrate and a second functional group having a first ionic chargemoiety on a surface of the self-assembled monolayer.
 4. The structure ofclaim 4, wherein the CNT layer comprises a second ionic charge moiety,and the first ionic charge moiety and the second ionic charge moiety areoppositely charged.